In one or more embodiments, this disclosure is directed to a system and method useful for providing variable negative receiver gain compensation when using an antenna with a pre-amplifier, and for improving the overall system Noise Figure (NF) in a time-division duplex (TDD) communication system. In other embodiments, this disclosure is directed to a variable gain circuit configured to receive a TDD communication signal from a pre-amplified antenna unit at a first power level and to output the communication signal to a remote radio head (RRH) at a second power level different than the first power level. The variable gain circuit may be retrofitted into an “off-the-shelf” pre-amplified antenna unit.
The ever-increasing size of wireless networks has led to the development of new communication system architectures. Traditional telecommunication base stations have baseband and radio-frequency (RF) components mounted inside an air-conditioned hut with co-axial cables transmitting signals to remote antennas. Conventionally, antennas were not provided with amplification before the signals are transmitted for further processing.
However, the wireless industry is moving away from base station architectures to distributed network and Remote Radio Head (RRH) architectures where the baseband components are digitally connected to a group of RF components mounted on top of antenna towers. This approach reduces the RF power requirements from the power amplifier (PA) and improves signal transfer through the use of fiber-optic cables. In addition, a central base station connected to multiple antennas eliminates the need for the redundancy of baseband units, reducing both capital and maintenance costs.
FIG. 1A provides a simplified representation of a conventional communication system 100 feeding an RRH without a pre-amplified antenna unit. Conventional communication system 100 includes tower-top antenna system 110 sitting atop tower 120. Tower top antenna system 110 includes antenna unit 130 with multiple antennas 131n and 132n. In one exemplary implementation of antenna unit 130 with n=10, 20 antenna elements may be used to form two “phase-matched” antenna arrays—a first antenna array that includes the “−45°” elements (i.e., antenna elements 1311-13110), and a second antenna array that includes the “+45°” elements (i.e., antenna elements 1321-13210). First and second phase-matched array outputs of antenna unit 130 are provided to RRH 140 which provides a communication signal to a baseband unit in a ground station (not shown) attached to node “A”. Power may be provided via node “B” to RRH 140 and antenna unit 130 from the ground station. Antenna unit 130 may comprise bar, dipole, patch, parabolic, dish, array, or some other type of antenna. Different arrays may be used, i.e., where n>10, or n<10.
FIG. 1B illustrates further details of conventional communication system 100 for a receive path in which antenna elements having the same phase, e.g., antenna elements 131n and 131n+1 (i.e., two “−45°” elements) are combined in two-way power combiner 150, provided over RF cable 152 to phase-shifter 160, and combined with similar signals from remaining combinations of antenna elements 131n and 131n+1 in 5-way power combiner 170. RF cable 172 connects an output of 5-way combiner 170 to connector 180 which is further connected to jumper cable 182 for application to an input of RRH 140. FIG. 1B also provides representative losses (in decibels) for each component in conventional communication system 100. In this example, the combination of losses contribute to an overall noise figure (NF) of 4.6 dB. For simplicity, the second antenna array that includes the “+45°” elements (i.e., antenna elements 1321-13210) is not shown, but it should be understood that these elements may be arranged in a “mirror image” fashion similar to that for the “−45°” elements (i.e., antenna elements 1311-13110).
Table I below provides a representative calculation using the known Friis Equations for determining the NF of conventional communications system 100 that uses a non-amplified or “plain” antenna unit 130.
TABLE I
This NF of 4.6 dB may not be suitable for some system requirements.
Another more recent conventional approach uses an antenna product by KMW Communications, Inc., originally configured for use in base station architectures. This antenna product is known as the “LNA+”, and incorporates an antenna arrangement with an integral built-in Tower-Top Low Noise Amplifier (TTLNA or LNA) behind each active pair of antenna elements to reduce losses and improve the receiver's NF. By placing amplifiers immediately behind the active elements inside the antenna to provide a “pre-amplified” antenna, jumper and phasing wiring harness losses do not contribute to net NF, providing an advantage over an external TTLNA used in conventional base station architectures and over the use of “plain” antennas as discussed above with respect to FIGS. 1A and 1B.
KMW's LNA+ is a full band TTLNA with vertical and horizontal azimuth steering capable antenna controlled by Antenna Interface Standards Group (AISG) protocol (e.g., version 2.0) with phase shifters all in one radome. “Redundant” LNA+'s are provided in each of 10 TTLNA modules. This technology is reported to improve uplink NF by balancing the transmit (TX) link with the larger footprint per cell, and improve uplink data through to existing cells. Because the components are inside one radome, extra cabling and water intrusion is eliminated, along with their associated losses and maintenance issues. This approach using a pre-amplified antenna eliminates the need for expensive high-gain antennas.
The LNA+ may be used in base station architectures. However, to operate within its design dynamic range (DR), a receiver using a pre-amplifier to overcome feedline losses in conjunction with a pre-amplified antenna as described above, e.g., a TTLNA or LNA used in base station receiver architectures, must incorporate a fixed value of receiver gain compensation (RGC) to avoid overdriving the receiver, as described in copending application Ser. No. 12/251,657 by Rausch et al. In this conventional application, it is important to protect receiver performance by constructing the LNA circuit such that only sufficient amplification is added to balance the loss in the feeder system between the LNA and the base station receiver without overdriving the receiver and degrading its DR. The RGC circuit may be “preset” by a technician to a fixed negative gain value appropriate for use between a particular base station receiver and pre-amplified antenna system Rausch et al. provides a novel circuit that, if installed in the bottom of the KMW LNA+ antenna, would allow a technician to “pre-set” negative receiver gain compensation at the antenna during installation to match the pre-amplified antenna to the particular base station receiver.
The relatively new LNA+ pre-amplified antenna approach described above can also be used in an RRH architecture, but with certain restrictions. For example, the RRH must be specifically built for that function, i.e., an RRH must be specifically configured and adapted for use with the particular pre-amplified antenna to avoid overdriving the RRH with too high a signal level from the pre-amplified antenna. Reconfiguration and adaptation of multiple RRHs that are found in conventional wireless radio transmission systems add expense and installation/update time, particularly given the large number of potentially different RRHs that may be necessary in a typical system RRH units from most current vendors do not incorporate an RGC, particularly one which prevents overdriving of its front-end by the pre-amplified antenna output.
FIG. 1C illustrates a receive path for conventional communication system 100′ similar to conventional communication system 100 of FIG. 1B in which phase matched antenna pairs 131n and 131n+1 may be combined in two-way power combiner 150, but which are then provided to LNA 190 for amplification before being sent over RF cable 152 to phase-shifter 160, and combined with similar signals from remaining combinations of antennas 131n and 131n+1 in 5-way power combiner 170. RF cable 172 connects an output of 5-way combiner 170 to connector 180 which is further connected to jumper cable 182 for application to an input of a modified RRH 140′. In this modified configuration, RRH 140′ has been customized by provision of an RGC to match the output power of LNA 190 to the desired input of RRH 140′ to keep RRH 140′ from being overdriven by LNA 190. In this example, the combination of losses contribute to an overall noise figure (NF) of 2.3 dB, a 2.3 dB improvement over the noise figure performance of conventional system 100.
FIG. 1C only illustrates a receive mode of operation, but system 100′ actually is bi-directional, i.e., RRH 140′ may be a transceiver that is capable of both transmitting and receiving RF signals, e.g., TDD signals. RRH 140′ may be configured to provide bypass command 195 during a transmit cycle where relatively high power RF signals are prevented by bypass command 195 from interfering with the receive input of RRH 140′ and the signal input to LNA 190.
Table II below provides a representative calculation for the NF of conventional communications system 100′ using antenna unit 130 with pre-amplifier (e.g., LNA 190) and customized RRH 140′, as illustrated in FIG. 1C.
TABLE II
However, even if RRH 140′ unit does include such an RGC and improves the overall system NF, the RGC approach identified above only partially solves problems in RRH architectures. System integrators are faced with the challenge of interfacing pre-amplified antenna systems such as the LNA+ with different types of RRH units whose developers/vendors would be burdened with an additional circuit configuration simply to satisfy the needs of one antenna system supplier. Furthermore, noise Figure improvement alone may not be the determining factor in deciding on the implementation details for a communication system.
What is needed is a system and method for modifying a pre-amplified antenna system such that it is compatible with a variety of different RRH units. What is further needed is a system and method for providing an adjustable receiver gain circuit associated with a pre-amplified antenna system such that any vendor's RRH may be used with a pre-amplified antenna. What is even further needed is a system and method for providing a negative gain amplifier used in conjunction with a pre-amplified antenna system such that any vendor's RRH may be used to prevent overdriving the RRH front-end and loss of DR, without requiring any changes to be made to an “off-the-the-shelf” RRH.